1. Field of the Invention
This invention relates to an implantable medical device that delivers sufficient electrical energy to cardiac tissue to defibrillate or cardiovert tachyarrhythmias and thus restore normal sinus rhythm. An improved DC-DC converter control circuit provides a biphasic shock.
2. Description of the Prior Art
Implantable medical devices for the therapeutic stimulation of the heart are will known in the art from U.S. Pat. No. 4,253,466 issued to Hartlaub et al, which discloses a programmable demand pacemaker. The demand pacemaker delivers electrical energy (5-25 microjoules) to the heart to initiate the depolarization of cardiac tissue. This stimulating regime is used to treat heart block by providing electrical stimulation in the absence of naturally occurring spontaneous cardiac depolarizations.
Another form of implantable medical device for the therapeutic stimulation of the heart is the automatic implantable defibrillator (AID) described in U.S. Pat. No. 27,757 to Mirowski, et al and the later U.S. Pat. No. 4,030,509 to Heilman et al. These AID devices deliver energy (40 joules) to the heart to interrupt ventricular fibrillation of the heart. In operation, the AID device detects the ventricular fibrillation and delivers a nonsynchronous high-voltage pulse to the heart through widely spaced electrodes located outside of the heart, thus mimicking transthoracic defibrillation. The Heilman et al technique requires both a limited thoracotomy to implant an electrode near the apex of the heart and a pervenous electrode system located in the superior vena cava of the heart.
Another example of a prior art implantable cardioverter includes the pacemaker/cardioverter/defibrillator taught by U.S. Pat. No. 4,375,817 to Engle et al. This device detects the onset of tachyarrhythmia and includes means to monitor or detect the progression of the tachyarrhythmia so that progressively greater energy levels may be applied to the heart to interrupt a ventricular tachycardia or fibrillation.
A further example is that of an external synchronized cardioverter, described in "Clinical Application of Cardioversion" in Cardiovascular Clinics, 1970, Vol. 2, pp. 239-260 by Douglas P. Zipes. This external device provides cardioversion shocks synchronized with ventricular depolarization to ensure that the cardioverting energy is not delivered during the vulnerable T-wave portion of the cardiac cycle.
Still another example of a prior art implantable cardioverter includes the device disclosed in U.S. Pat. No. 4,384,585 to Douglas P. Zipes. This device includes circuitry to detect the intrinsic depolarizations of cardiac tissue and includes pulse generator circuitry to deliver moderate energy level stimuli (in the range of (0.1-10 joule) to the heart synchronously with the detected cardiac activity.
The functional objective of this stimulating regimen is to depolarize areas of the myocardium involved in the genesis and maintenance of re-entrant or automatic tachyarrhythmias at lower energy levels and with greater safety than is possible with nonsynchronous cardioversion. Nonsynchronous cardioversion always incurs the risk of precipitating ventricular fibrillation and sudden death. Synchronous cardioversion delivers the shock at a time when the bulk of cardiac tissue is already depolarized and is in a refractory state. Other examples of automatic implantable synchronous cardioverters include Charms U.S. Pat. No. 3,738,370.
It is expected that the safety inherent in the use of lower energy levels, the reduced trauma to the myocardium, and the smaller size of the implanted device will expand the indications for use for this device beyond the patient base of prior art automatic implantable defibrillators. Since many episodes of ventricular fibrillation are preceded by ventricular (and in some cases, supraventricular) tachycardias, prompt termination of the tachycardia may prevent ventricular fibrillation.
Consequently, current devices for the treatment of tachyarrhythmias include the possibility of programming staged therapies of antitachycardia pacing regimens, along with cardioversion energy and defibrillation energy shock regimens in order to terminate the arrhythmia with the most energy efficient and least traumatic therapies (if possible). In addition, current devices envisage inclusion of single or dual chamber bradycardia pacing therapies, all of which are described, for example, in U.S. Pat. No. 4,800,833 to Winstrom, U.S. Pat. No. 4,830,006 to Haluska et al, and U.S. patent application Ser. No. 07/612,758 filed Nov. 14, 1990, and incorporated herein by reference.
Furthermore, as described in the '833 and '006 patents and the '758 application, considerable study has been undertaken to devise the most efficient electrode systems and shock therapies.
Initially, implantable cardioverters and defibrillators were envisioned as operating with a single pair of electrodes applied on or in the heart. Examples of such systems are disclosed in the aforementioned '757 and '509 patents wherein shocks are delivered between an electrode placed in or on the right ventricle and a second electrode placed outside the right ventricle. Studies have indicated that two electrode defibrillation systems often require undesirably high energy levels to effect defibrillation. In an effort to reduce the amount of energy required to effect defibrillation, numerous suggestions have been made with regard to multiple electrode systems. For example, sequential pulse multiple electrode systems are disclosed in U.S. Pat. No. 4,291,699, issued to Geddes, et al, in U.S. Pat. No. 4,708,145, issued to Tacker, et al, in U.S. Pat. No. 4,727,877 issued to Kallock and in U.S. Pat. No. 4,932,407 issued to Williams. Sequential pulse systems operate based on the assumption that sequential defibrillation pulses, delivered between differing electrode pairs have an additive effect such that the overall energy requirements to achieve defibrillation are less than the energy levels required to accomplish defibrillation using a single pair of electrodes.
An alternative approach to multiple electrode, sequential pulse defibrillation is disclosed in U.S. Pat. No. 4,641,656 issued to Smits and also in the above-cited '407 patent. This defibrillation method may conveniently be referred to as multiple electrode, simultaneous pulse defibrillation, and involves the delivery of defibrillation pulses simultaneously between two different pairs of electrodes. For example, one electrode pair may include a right ventricular electrode and a coronary sinus electrode, and the second electrode pair may include a right ventricular electrode and a subcutaneous patch electrode, with the right ventricular electrode serving as a common electrode to both electrode pairs. An alternative multiple electrode, single path, biphasic pulse system is disclosed in U.S. Pat. No. 4,953,551 issued to Mehra, et al, employing right ventricular, superior vena cava and subcutaneous patch electrodes.
In the above-cited prior simultaneous pulse, multiple electrode systems, delivery of the simultaneous defibrillation pulses is accomplished by simply coupling two of the electrodes together. For example, in the above-cited '551 patent, the superior vena cava and subcutaneous patch electrodes are electrically coupled, and a pulse is delivered between these two electrodes and the right ventricular electrode. Similarly, in the above-cited '407 patent, the subcutaneous patch and coronary sinus electrodes are electrically coupled together, and a pulse is delivered between these two electrodes and a right ventricular electrode.
The above incorporated '758 application discloses a pulse generator for use in conjunction with an implantable cardioverter/defibrillator which is capable of providing all three of the defibrillation pulse methods described above, with a minimum of control and switching circuitry. The output stage is provided with two separate output capacitors, which are sequentially discharged during sequential pulse defibrillation and simultaneously discharged during single or simultaneous pulse defibrillation. The complexity of these stimulation therapy regimens require rapid and efficient charging of high voltage output capacitors from low voltage battery power sources.
Typically, the electrical energy to power an implantable cardiac pacemaker is supplied by low voltage, low current, long-lived power source, such as a lithium iodine pacemaker battery of the types manufactured by Wilson Greatbatch Ltd. and Medtronic, Inc. While the energy density of these power sources is relatively high, they are not designed to be rapidly discharged at high current drains, as would be necessary to directly cardiovert the heart with cardioversion energies in the range of 0.1-10 joules. Higher energy density battery systems are known which can be more rapidly discharged, such as lithium thionyl chloride power sources. However, none of the available implantable, hermetically sealed power sources have the capacity to directly provide the cardioversion energy necessary to deliver an impulse of the aforesaid magnitude to the heart following the onset of tachyarrhythmia.
Generally speaking, it is necessary to employ a DC-DC converter to convert electrical energy from a low voltage, low current power supply to a high voltage energy level stored in a high energy storage capacitor. A typical form of DC-DC converter is commonly referred to as a "flyback" converter which employs a transformer having a primary winding in series with the primary power supply and a secondary winding in series with the high energy capacitor. An interrupting circuit or switch is placed in series with the primary coil and battery. Charging of the high energy capacitor is accomplished by inducing a voltage in the primary winding of the transformer creating a magnetic field in the secondary winding. When the current in the primary winding is interrupted, the collapsing field develops a current in the secondary winding which is applied to the high energy capacitor to charge it. The repeated interruption of the supply current charges the high energy capacitor to a desired level over time.
In U.S. Pat. No. 4,548,209 to Wielders et al, as well as in the above-referenced '883 patent, charging circuits are disclosed which employ flyback oscillator voltage converters which step up the power source voltage and apply charging current to output capacitors until the voltage on the capacitors reaches the programmed shock energy level.
Specifically, in the charging circuit 34 of FIG. 4 of the '209 patent, the two series connected lithium thionyl chloride batteries 50 and 52 are shown connected to the primary coil 54 of transformer 56 and to the power FET transistor switch 60. The secondary coil 58 is connected through diode 62 to the cardioversion energy storage capacitor 64. Very generally, the flyback converter works as follows: When switch 60 is closed, current I.sub.p passing through the primary winding 54 increases linearly as a function of the formula V=L.sub.p dI/dt. When FET 60 is opened, the flux in the core of the transformer 56 cannot change instantaneously so a complimentary current I.sub.s which is proportional to the number of windings of the primary and secondary coils 54 and 58 respectively starts to flow in the secondary winding 58 according to the formula (N.sub.p /N.sub.S)I.sub.p. Simultaneously, voltage in the secondary winding is developed according to the function V.sub.s =L.sub.s dU/dt.sub.s. The cardioversion energy storage capacitor 64 is charged thereby to the programmed voltage.
The power FET is switched "on" at a constant frequency of 32 KHz for a duration or duty cycle that varies as a function of the voltage of the output capacitor reflected back into the primary coil 54 circuit. The on-time of power FET 60 is governed by the time interval between the setting and resetting of flip-flop 70 which in turn is governed either by the current I.sub.p flowing through the primary winding 54 or as a function of a time limit circuit, which contains further circuitry to vary the time limit with battery impedance (represented schematically by resistor 53). In both cases, the on-time varies from a maximum to a minimum interval as the output circuit increases to its maximum voltage.
The '883 and '006 patents disclose a variable duty cycle flyback oscillator voltage converter, where the current in the primary coil circuit (in the case of the '883 patent) or the voltage on a secondary coil (in the case of the '006 patent) is monitored to control the duty cycle of the oscillator. In the '883 circuit the "on" time of the oscillator is constant and the "off" time varies as a function of the monitored current through the transformer.
In the '006 patent, a secondary coil is added to power a high voltage regulator circuit that provides V+ to a timer circuit and components of the high voltage oscillator. This high voltage power source allows the oscillator circuit to operate independently of the battery source voltage which may deplete over time. The inclusion of a further secondary winding on an already relatively bulky transformer is disadvantageous from size and efficiency standpoints.